Implantable medical devices for remedial treatment of and compensation for cardiac, neural and muscular disorders, are known in the art. These devices range from cardiac pacemakers as described in U.S. Pat. No. 4,712,555 to Thornander et al., to microstimulators as described in U.S. Pat. No. 6,208,894 to Schulman et al. The quest for minimization of such devices, especially in the area of microdevices such as microstimulators and microsensors, has presented the challenge of efficiently providing a reliable and stable power source to power the device or charge its internal battery as well as a communication medium to facilitate information, data and command signal transfer and exchange between the microdevice and a corresponding microdevice control unit. Heretofore, wireless communication between the control unit and the implanted device including microdevices, has been described as being implemented by means of a modulated sound signal, time varying (such as an alternating current AC) magnetic field or light source. In certain instances, wireless communication, which is also intended as a means of power delivery to an implanted microdevice, is by way of a time varying magnetic field generated by an inductor positioned in proximity to the microdevice. The inductor may be formed on a flexible support which contains a series of closely wound electrically conductive wires that when energized, generate a magnetic field in the vicinity of such wires. Utility is found in the use of such flexible support when it is desired to bring the inductor in close proximity with microdevices that are implanted in regions of the body characterized as being very contoured. For example, in the instance when microdevices are implanted on either side of a patient's neck, the inductor and therefore the inductor support must be sufficiently flexible and pliable to permit wrapping the inductor around the patient's neck so that the inductor will simultaneously be in proximity with all the implanted microdevices in the neck. The net result of the inductor being so positioned, is that maximum magnetic coupling is theoretically achieved between the inductor and microdevice, so that communication whether it is intended for example, for data transfer or charging a rechargeable microdevice battery, will be most efficiently and reliably realized. Data transmission between such devices may involve the use of magnetic field modulation techniques using known data transmission protocols.
The basis for good wireless communication in cases where magnetic field coupling is the technique of choice, is that the magnetic field strength should be unaffected by manipulation of the inductor support as well as the introduction of magnetic field altering implements in the vicinity of the inductor. In such cases the effective inductance value of the inductor is observed to change. This can occur if the inductor is bent or distorted when applied to fit the contour of a desired location of a body. In systems that use a power amplifier driven tuned circuit comprising an inductor and capacitors, the maximum power delivered to the tuned circuit and therefore the maximum magnetic field strength produced by the inductor, occurs when the resonant frequency of the tuned circuit matches a reference frequency, that is applied to the power amplifier, and which usually relates to the nominal values of the capacitors and the inductor. Changes in inductance value of the inductor may have a severe impact on the resonant frequency of the tuned circuit and therefore have a correspondingly negative effect on any magnetic field generated by the inductor. The fallout from such deterioration of magnetic field strength would be the deterioration of communication integrity between the microdevice and its control unit. It has been observed that inductor shape changes from circular to a flattened oval can result in a reduction of inductance value of as much as 50%. Since such inductance value changes may be dynamic in nature, what is needed is an automatic tuning system that dynamically and adaptively adjusts the resonant frequency of the tuned circuit and maintain it equal to the reference frequency.